When I turn the center knob to activate the shower(Price Phfister, circa 1969) the water does not fully shut-off at the main nozzle, only a slight amount exits the shower head.

Removed the control valve and replaced with another with the same results.

If you've replaced the shower valve and still have a problem then perhaps the supplies are loaded with crud that's fouling the valve. Did you flush out the supplies when you had everything apart? Let me know, tom